An instrument used for measuring AC electric signals typically has a high impedance front end that substantially isolates the instrument from the circuit containing the signal being measured. The isolation allows the instrument to make an accurate measurement without significantly disturbing the signal being measured. Conversely, if the instrument were to have a low impedance front end, the instrument would not be isolated from the circuit and the signal being measured would be loaded down by components in the instrument, thereby resulting in an inaccurate measurement of the signal.
In many instances, the front end these instruments includes a high input impedance AC buffer, which consists, in part, of an operational amplifier. If the instrument is intended to measure a broad range of signal magnitudes, the front end will undoubtedly have a range selection capability. The range selection capability in prior art instruments is provided by resistors selectably connected in series with the input signal path ahead of the operational amplifier or in the feedback path of the buffer. When connected in the signal path, the selected resistor(s) scales the signal magnitude as required by the instrument. When connected in the feedback path, the selected resistor(s) determine the gain of the buffer. Traditionally, resistors having high resistance values have been used in high input impedance AC buffers for both signal scaling and feedback gain control. The high resistance of the input resistors provides the high impedance characteristic of these AC buffers.
As is well known by one having ordinary skill in the amplifier art, the gain of the buffer must remain substantially constant for all frequencies of input signals that are to be accurately measured. A signal whose frequency lies in that portion of the buffer's bandwidth in which the gain is not constant, that is, where the buffer gain is changing, will not be accurately measured by the instrument. Unfortunately, the combination of the high resistance of the prior art input and feedback resistors and the stray capacitance associated with the buffers cause the gain of the buffer to vary. Typically this gain variation occurs for signal frequencies in the upper end of the bandwidth of the buffer. To correct this problem, prior art instruments must be compensated so as to be calibrated at these high signal frequencies.
Historically, calibration of prior art instruments has been accomplished by inserting compensation capacitors in the buffer circuit. In effect, these capacitors reduce the effects of stray capacitance of the buffer. The compensation capacitors may include adjustable type capacitors. Depending on the sophistication of the instrument, the calibration procedure may be performed manually or automatically. When performed manually, a trained technician removes the instrument cover and adjusts the capacitors until the instrument falls within the manufacture's specifications. Such a procedure can be rather time consuming. If the instrument is of the type that is calibrated automatically, the procedure is typically performed by a calibration program embedded in the instrument. The program instructs switches to open and close so that fixed compensation capacitors are connected to the buffer circuit in a predetermined manner. Obviously, the complexity of the calibration program increases as the number of capacitors and switching operations increase.
As is readily apparent from the foregoing discussion, manual calibration of prior art signal measuring instruments is time consuming and, therefore expensive, while automatic calibration procedures add to the complexity and expense of the instruments. Furthermore, the compensation capacitors used in prior art instruments increase the manufacturing cost of the instrument. Yet another problem caused by the use of compensation capacitors is the corresponding increase in the number of components, which hampers efforts to reduce the size of the instrument.
Accordingly, there is a need for a high input impedance AC buffer suitable for use in an instrument that does not require calibration at high signal frequencies. The buffer should have as few parts as possible so that it is compact and inexpensive to manufacture. The present invention is directed to an input buffer having lower feedback resistance designed to achieve these results.